Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. Further, one or more intake charging devices may be staged in series or parallel to an intake turbocharger to improve turbocharged engine boost response.
One example of a multi-staged intake charging system is shown by Stewart in U.S. Pat. No. 7,958,730. Therein, a high pressure turbine is staged upstream of a low pressure turbine, each turbine coupled to a corresponding compressor. The multi-staged configuration allows for multiple degrees of freedom in the boosted engine system, thereby enabling the control of two set-points, one of which includes boost pressure.
However, the inventors herein have identified potential issues with such multi-staged systems. As one example, turbocharged engine systems may have several hardware limits, such as a maximum turbocharger shaft speed, which could be violated under high engine load or when operating a vehicle at high altitude. As such, turbocharger speed limits cannot be exceeded due to the potential for substantially immediate mechanical degradation. Current control systems may address this issue by clipping the maximum boost pressure when such a constraint violation is anticipated. Additionally, airflow actuators may be adjusted to reduce the boost pressure, such as by opening a wastegate and/or a compressor recirculation valve. However, the drop in boost output below the driver demanded boost pressure may result in a noticeable under-delivery of torque demand, and a drop in vehicle driveability. In addition, the vehicle operator's drive experience is degraded.
In view of these issues, a method for improving turbocharger component speed control in a boosted engine having multiple, staged charge boosting devices is provided. The method includes: bypassing a second compressor and providing a flow of compressed air to a piston engine via a first compressor; and in response to a shaft speed of the first compressor being at or above a threshold, accelerating the second compressor. In this way, turbocharger speed control is enabled without degrading boosted engine performance.
As one example, a boosted engine system may include an electric supercharger coupled upstream of a turbocharger. For example, the supercharger may be coupled upstream of a charge air cooler. During conditions when boost is required and while the turbine is spinning up, the electric supercharger may be used to provide compressed air to the engine. Then, once the turbine spins up, the turbocharger compressor may be used to provide compressed air to the engine, while bypassing the supercharger. A turbocharger shaft speed may be monitored based on, for example, the turbine speed or the turbocharger compressor speed. If the turbocharger compressor shaft reaches a speed limit (or if the turbocharger compressor or turbine speed reaches the speed limit) while the demanded boost pressure is provided by the turbocharger, the electric supercharger may be spun up to reduce the load on the turbocharger. Specifically, the supercharger may be activated to share part of the total required power. Due to the load sharing between the turbocharger and the supercharger, the turbocharger operating speed is reduced. The supercharger compressor may be spun via one or more of an electric motor and the engine crankshaft at a speed that is based on the turbocharger shaft speed, the supercharger compressor speed increased as the turbocharger shaft speed exceeds the speed limit. In addition, a ratio of power delivered to the supercharger via the electric motor relative to the crankshaft may also be adjusted based on the turbocharger shaft speed. Once the turbocharger speed has been controlled, the supercharger may be disabled, and compressed air may once again be provided via the turbocharger.
The technical effect of sharing the boost load of a first, downstream turbocharger compressor via operation of a second, upstream supercharger compressor is that turbocharger compressor over-speed can be addressed without degrading boosted engine performance. By operating the supercharger to decrease the boost load provided by the turbocharger, a speed of the first compressor can be reduced without the need for reducing boost pressure at the first compressor via operation of a compressor recirculation valve or wastegate. By reducing the turbocharger speed, component life is extended. By using the supercharger to both reduce the turbocharger speed and maintain the boost pressure, under-delivery of torque demand is averted, and vehicle driveability is not degraded. Overall, the performance of a boosted engine system having staged charging devices is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.